In general, the density of the earth increases with depth, from a lighter silicic crust (or basalt in the oceans) to a sequence of denser, more metallic rocks from the mantle to the inner core. This is what we expect when digging or drilling into the earth. But occasionally we are surprised by the discovery of lighter, less dense earth layers beneath denser ones. Sometimes these layers are actually occupied by nothing but air or water and they surprise us in the form of weak rock zones, sinkholes and voids, which can cause great mischief for us and our infrastructure projects including roadways, pipelines, buildings and other projects that depend on a solid earth foundation.
Fortunately, modern surface geophysical techniques can be used to help identify areas where subsurface voids are located so that they can be avoided at the design phase of a project. The primary tools at the geophysicists’ disposal are electrical resistivity, seismic profiling, and ground penetrating radar. Each of these techniques has unique capabilities in identifying voids and weak rock zones.
If voids are located within several feet of the ground surface. Ground Penetrating Radar (GPR) can be an effective tool. The radar waves will show a significant reflection at the interface of the overlying denser and underlying less dense materials (or void). As depth increases (10’+) other geophysical techniques can be successfully applied. Using Electrical Resistivity Tomography (ERT), changes in density can be found by processing the apparent electrical resistivity at various depths (up to 100′) and locations. Using interpretive software the profile of underlying densities can be approximated.
Seismic data, involving the transmission of sonic waves through the subsurface, can also be processed with appropriate software to allow interpretation of underlying earth densities. There are certain instances where seismic is incapable of accurately imaging below density inversions but the technique can be effective to the point of the inversion. In the oil and gas industry, the use of seismic methods led to one of the major breakthroughs in the discovery of natural gas by revealing so called ‘bright spots’ in the data which typically highlight areas where less dense gas filled porous rocks underlay overlying denser materials.
Using the methods described above, when applied appropriately for the site conditions and objectives, can help architects, engineers, land planners and designers, geologists and construction professionals identify potential areas of poor subsurface conditions, saving significant time and money. Geophysical capabilities are continually improving through software and hardware improvements, with 3D and GPS advances making the acquisition and interpretation more accurate and understandable. Stay tuned.
Example of Shallow Sub-Slab Voids Identified in a GPR Survey (~3 Feet Below Ground Surface)
Example of Electrical Resistivity Survey Identifying Karst/Sinkhole Conditions
Example of Seismic Results Showing Density and Depth Variability in a Rock Formation